British authorities admitted mistakes in their investigation of three schoolgirls who left the United Kingdom to join ISIS, after their families complained police should have contacted them during an earlier investigation into the disappearance of a classmate.

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Scotland Yard had met with Shamima Begum, Kadiza Sultana, and Amira Abase about the disappearance of a classmate at their school, the Bethnal Green Academy. The girls were given letters by investigators to take home that asked their parents for permission to take statements, but the girls hid the letters, the BBC reports.

Halima Khanon, Sultana's sister, told the BBC that if the family knew about the investigation, they would have "taken precautions with my sister."

In a statement, police said the parents were told about the meetings by a school official, and that when they met with the girls, they were being treated as witnesses in the case of their missing friend. Police said there was no reason to believe the girls planned to travel to Syria.

"With the benefit of hindsight, we acknowledge that the letters could have been delivered direct to the parents," the police said in a statement.

Courtesy of BBC

CCTV footage reportedly captured three British schoolgirls in Istanbul as they headed to Syria to join ISIS on Feb. 17, 2015.

Begum and Abase, both 15, and Sultana, 16, were last seen February 17 when they boarded a flight from London's Gatwick Airport headed for Turkey.

Surveillance video obtained by the BBC shows the girls waiting in a bus station for about 18 hours. They then boarded a bus to Gaziantep, a Turkish border town that has gained notoriety for being portal for foreign fighters traveling to Syria.

ABC News' Alexander Mallin and The Associated Press contributed to this story.